It is often desirable to evaluate the properties of a material without damaging the specimen being tested. Several known techniques make use of magnetism to evaluate specific properties in a specimen sample. Such evaluation techniques include eddy-current testing, magnetic particle inspection and flux leakage inspection. Such techniques, however, provide only limited information regarding the properties of the material under evaluation.
Magnetic particle inspection and flux leakage inspection represent well known magnetic evaluation techniques. These techniques permit detection of cracks and pitting due to the perturbance of magnetic flux caused by such inhomogeneities. These techniques, however, are unsuitable for the identification and measurement of discontinuities in material properties such as stress or fatigue which produce subtle changes in the characteristics of a material.
The ability to obtain more meaningful information regarding a material's less pronounced intrinsic properties through measurement of magnetic properties has been recently recognized by those skilled in the art. For example, it is known that coercivity can be used to detect plastic deformation and hardness, that maximum differential permeability can be used to measure stress, that remanence can be used to detect creep damage, that a combination of remanence and coercivity can be used to detect impending fatigue failure and that hysteresis loss can be used to detect changes in grain boundary segregation arising from temper embrittlement.
This ability to correlate the magnetic characteristics of a material to its less pronounced intrinsic physical properties is disclosed in U.S. Pat. Nos. 5,008,621 and 5,012,189 both of which issued to the present inventor.
As recognized in the art and taught by both the '621 patent and the '189 patent, bulk magnetic properties such as coercivity, remanence, hysteresis loss, initial permeability, maximum differential permeability and anhysteretic permeability may be derived from magnetic hysteresis curves. As is well known, the magnetic hysteresis curve is a plot of flux density B in a material versus a varying applied magnetic field intensity H. Both the '621 patent and the '189 patent further teach that information regarding the physical properties of a sample specimen may be obtained from the evaluation of the magnetic properties occurring therein.
The '189 patent entitled "Method for Deriving Information Regarding Stress from a Stressed Ferromagnetic Material" is illustrative of one type of physical data which can be derived from magnetic measurements. Specifically, the '189 patent discloses the ability to derive a meaningful estimate of the actual and residual stress in a material based on the hysteresis and anhysteretic magnetization curves at the origin as compared to such curves in an unstressed sample.
The system and method disclosed in U.S. Pat. No. 5,059,903 to Otaka et al. illustrates another type of physical data which may be derived from magnetic measurements. As disclosed in the Otaka patent, the embrittlement of a material can be evaluated through comparison of the magnitude of magnetization characteristics measured in a sample specimen to the magnetic characteristics of a virgin specimen. By making these comparative analyses on a periodic basis, it is possible to determine degradation rates as well as to identify areas in which degradation is most severe.
While useful in evaluating a specific property of the specimen, namely embrittlement, it will be appreciated that the Otaka patent does not provide for the evaluation of a broad scope of material properties such as hardness, internal stress or grain boundary segregation which may be of interest.
The system and method disclosed in the '621 patent to the present inventor provides a useful means to evaluate the bulk magnetic properties occurring in a sample as a whole without the need to make a comparison to virgin material as is required by the Otaka patent. As disclosed in the '621 patent, this bulk analysis is effected by subjecting an entire sample to a variable magnetic field and taking multiple measurements of the magnetic field and the magnetic flux of the specimen as the magnetic field is cycled in a controlled manner. As will be recognized, such a controlled cycling permits the collection of data sufficient to generate a hysteresis curve.
The system and method of the '621 patent are useful in generating information relative to the sample as a whole. However, due to the need to surround the sample with a magnetic field, no information is obtained regarding differences in a material's magnetic properties, and hence physical characteristics, occurring across the specimen surface. Further, the '621 patent incorporates no means to display data relating to the magnetic properties occurring across an area of the sample specimen.